When a plurality of metal lines are formed by a metal film in the same layer, the metal lines might cross each other. In such a case, the crossing of two metal lines can be avoided by coupling one of the two metal lines to a metal line formed by a metal film in another layer. Coupling a metal line in one layer to a metal line in another layer as above requires the metal line that is made of a metal film in one layer and the metal line that is made of a metal film in another layer to be connected to a connection line made of a metal film in still another layer.
However, depending on the combination of metal layers included in the metal lines and the connection line, contact faults due to electric corrosion might occur where the metal lines and the connection line contact each other. FIG. 15 provides views illustrating a conventional line connection structure 250; more specifically, FIG. 15(A) is a cross-sectional view of the conventional line connection structure 250, and FIG. 15(B) is a plan view of the conventional line connection structure 250. As shown in FIGS. 15(A) and 15(B), a first metal line 222, which is made of a film stack of an aluminum (Al) layer and a molybdenum (Mo) layer provided thereon, is formed on an insulating substrate 201 such as a glass substrate. Formed on the first metal line 222 is a gate insulating film 203 covering the first metal line 222 on each end portion.
On the gate insulating film 203 formed on one end portion, a second metal line 225 is formed of, for example, a film stack of an aluminum (Al) layer 225a and a molybdenum nitride (MoN) layer 225b provided thereon, and further, on the second metal line 225, a passivation layer 207 and an organic insulating film 208 are stacked. On the gate insulating film 103 formed on the other end portion, no second metal line 225 is formed, and the passivation layer 207 and the organic insulating film 208 are simply stacked. In this manner, the passivation layer 207 and the organic insulating film 208 surround an area provided as a contact hole 240.
Formed on an inner surface of the contact hole 240 is an ITO (indium tin oxide) layer 209 made of a transparent metal. By the ITO layer 209, the aluminum layer 225a, which is included in the second metal line 225 and protrudes into the contact hole 240, and the first metal line 222 are electrically connected via an opening 203c provided in the gate insulating film 203. As a result, the ITO layer 209 can serve as a connection line to couple the first metal line 222 and the second metal line 225.
Furthermore, Patent Document 1 discloses that either a hydrogen plasma treatment or an argon plasma treatment is performed on an IGZO (InGaZnO: indium gallium zinc oxide) film, which is an oxide semiconductor, thereby rendering the IGZO film conductive.